Brief Answer:
I would be happy to help, thanks for using HCM
Detailed Answer:
There are several possibilities, and she really needs to see a doctor to have it checked to figure out what is is. Fortunately, it is usually treatable. Some of the possibilities include: the result of trauma to the area (she may not be telling you that she masturbates, or she may have been struck in the area in some other way; It could be a swollen gland in the area, which can be very painful, but can usually be drained; It could be an infection from being cut when shaving the area, or it may be a skin reaction to an irritant. These are some of the things that I can think of. It is usually easy to diagnose on exam, so I really suggest that she be seen by her doctor. I hope this helps, please let me know if you have any more questions.